It is well known that progressiveness of the damping characteristic of vehicle suspension is a highly desirable feature that substantially improves shock absorption transmitted from the road wheels to the body of a vehicle. It is also well known that progressive rate vehicle suspensions with smooth (i.e. differentiable) damping characteristic commonly in use are pneumatic and hydro-pneumatic ones. However, these suspensions are inferior to steel ones in many aspects such as strength, durability, reliability and cost, and their damping characteristic, being determined by the thermodynamic properties (adiabatic exponent) of the gas (air or nitrogen) they utilize, cannot be freely adjusted and is far from optimum.
There are also some progressive rate vehicle suspensions fitted with steel springs, but they usually feature inferior non-differentiable damping characteristic.
The problem of constructing purely mechanical steel progressive rate vehicle suspension has been undertaken by many inventors (see U.S. Pat. Nos. 3,157,394, 4,010,941 and International Publication WO-A-96 11815 of the International Application PCT/CA 95/00570), but none of such suspensions proposed in the past was a success. This is due to the fact that those suspensions used unreliable and perishable cam mechanisms to achieve required non-linearity of damping characteristic.
Recently, a very satisfactory solution to the problem of achieving progressive steel vehicle suspension was proposed (U.S. Pat. No. 6,851,690, European Patent 1,210,236, HK Patent 1,033,149, Polish Patent 192,322). This suspension uses an innovative, extraordinarily robust and compact 4-link mechanism (the strongest mechanism in existence) and is capable of producing very favorable progressive rate differentiable damping characteristic out of linear characteristic of ordinary (e.g. steel) springs of any kind. Moreover, the damping characteristic can be freely shaped and adjusted to any specific requirements (in particular suspension with optimal exponential-like damping characteristic can be constructed).
However, achieving steel springs with differentiable progressive spring rate (by a method as simple as possible) is still of great interest, as applying such springs in vehicle suspensions would result in simpler, lighter, more compact and cheaper vehicles. Moreover, progressive rate springs could find many other applications (aircraft landing gears, foundations of heavy machines (hammers, presses, engines) with highly improved vibrations damping capability, buffers with improved energy absorbtion capability, earth quake-proof building foundations etc.).
Some non-linear (progressive) rate steel springs, e.g. coil and leaf springs, are well known from the prior art, however these springs are far from being completely satisfactory. Progresiveness of coil springs is achieved e.g. by altering the tightness of a coil's winding: first few windings of progressive coil spring are made tighter (and therefore of smaller stiffness), and then spaced wider (and therefore stiffer). This assures only unsatisfactory discrete (approximately) change of the spring rate. Namely, as a rising force is loading the spring, first both the tighter and wider spaced coils are being compressed simultaneously and the spring displays a (smaller) spring rate resulting from the spring rates of these two types of its coils. Only after the tighter coils have completed their movement (as the force is still rising) and only the wider spaced coils of the spring have remained active, the spring starts to display increased stiffness (greater spring rate). Another method of making coil springs progressive is to give them conical shape. This method also provides springs with unsatisfactory characteristic, which is not differentiable (essentially, the discussion above applies also in this case).
Also leaf springs can be made non-linear) progressive. Commonly, this is achieved by applying a number of in turn activating leafs or by altering the effective spring length (using suitably shaped surfaces supporting the ends of the leaf spring). Again, these methods of achieving progressiveness of steel springs are well known to be not satisfactory.
There are also some patented methods of making leaf springs progressive. Some of them utilize dependence of the leaf spring rate on its effective length (e.g. U.S. Pat. No. 6,435,485, GB Patents 1,245,636 and 2,135,752, DE Patent 3,431,793, also US Patent Application Publications US 2002/0167121 and US 2003/0122293). Another method, provided by GB Patents 1,212,411 and 1,212,412 granted to Ford Motor Company Limited, utilizes dependence of the leaf spring rate on the longitudinal tensile loads (there are some other later patents based on the same idea). Yet another method of achieving progressiveness of leaf spring is known from JP Patent 55,112,436. In principle, the method provided by this patent is also based on dependence of the leaf spring rate on the longitudinal tensile loads, only the method of generating tensile loads in the leaf spring differs from that provided by GB Patents 1,212,411 and 1,212,412.
Another kind of non-linear springs well known from prior art are springs made of elastic bodies, usually assuming general shape of solid cylinder or cylindrical pipe, with elastic portions formed between slits cut in said elastic bodies (see U.S. Pat. No. 5,062,619 granted to Masahide Sato and References cited in this Patent). The non-linearity of the spring characteristic is obtained by varying elasticity of said elastic portions. This resembles the situation met with in the case of coil springs, the tighter and wider spaced coils of which can be compared to the portions with varying elasticity, and the discussion of the work of non-linear coil springs presented above applies almost literarily also to these springs. Consequently, the characteristic of the spring in question is not differentiable (it is piecewise linear). In particular, the spring with exponential-like characteristic cannot be produced by this method, and the variety of spring characteristics achievable in this way is surely limited, thus preventing springs of this type from being optimized for given application. Moreover, there is no algorithm for designing the characteristic of such springs, which therefore is to be determined experimentally, which substantially increases cost.
Springs with non-linear characteristic, which are typically made of spring steel by cold forming (e.g. stamping) or by bringing it into a stressed condition and then rendering a portion of the spring substantially stress-free by local heat treatment are also known from prior art (see GB Patent 1,550,877). However these springs have very special characteristic that can be hardly predicted and designed and range of applications of them is limited (they are used e.g. in snap-switches).
The invented method differs from these known from prior art (see for example U.S. Pat. No. 5,062,619 and GB Patent 1,550,877) in that it enables producing non-linear springs with precisely predictable (unlike the springs of GB Patent 1,550,877) and smooth (unlike the springs of U.S. Pat. No. 5,062,619) characteristic.
Let us also note that known vehicle suspensions are relatively complex and expensive and some of them (e.g. those utilizing leaf springs) are bulky and heavy, and making vehicle suspension simpler lighter and cheaper is undoubtedly an important task.
Thus there is a need for lightweight springs with variable (progressive) spring rate depending differentiably on external load applied to the spring and capable of being adjusted to any specific requirements, as well as there is a need for lightweight vehicle suspensions of simple structure utilizing such springs. There is also a need for an effective method for designing (algorithm) and manufacturing such springs (with any pre-assigned characteristic) and suspensions.